JavaScript can benefit
from many of the same speed-optimization techniques that are used
in other languages, like C1,2
and Java. Algorithms and data structures, caching frequently used
values, loop unrolling and hoisting, removing tail recursion, and
strength-reduction techniques all have a place in your JavaScript
optimization toolbox. However, how you interact with the Document
Object Model (DOM) in large part determines how efficiently your
code executes.

Unlike other programming languages,
JavaScript manipulates web pages through a relatively sluggish API,
the DOM. Interacting with the DOM is almost always more expensive
than straight computations. After choosing the right algorithm and
data structure and refactoring, your next consideration should be
minimizing DOM interaction and I/O operations.

With most programming languages, you can trade space for time complexity
and vice versa.3 But on the web, JavaScripts
must be downloaded. Unlike desktop applications where you can trade
another kilobyte or two for speed, with JavaScript you have to balance
execution speed versus file size.

How
Fast Is JavaScript?

Unlike C, with its optimizing compilers that increase execution
speed and decrease file size, JavaScript is an interpreted
language that usually is run over a network connection (unless
you count Netscape's Rhino, which can compile and optimize
JavaScript into Java byte code for embedded applications4).
This makes JavaScript relatively slow compared to compiled
languages.5 However, most scripts
are usually so small and fast that users won't notice
any speed degradation. Longer, more complex scripts are where
this chapter can help jumpstart your JavaScript.

Resources
for Web Developers.Trials, Software, Downloads,
and More.

Design Levels

A hierarchy of optimization levels exists
for JavaScript, what Bentley and others call design levels.6
First comes the global changes like using the right algorithms
and data structures that can speed up your code by orders of
magnitude. Next comes refactoring that restructures code in
a disciplined way into a simpler, more efficient form7).
Then comes minimizing DOM interaction and I/O or HTTP requests.
Finally, if performance is still a problem, use local optimizations
like caching frequently used values to save on recalculation
costs. Here is a summary of the optimization process:

Choose the right algorithm and data structure.

Refactor to simplify code.

Minimize DOM and I/O interaction.

Use local optimizations last.

When optimizing your code, start at the
highest level and work your way down until the code executes
fast enough. For maximum speed, work at multiple levels.

Measure Your Changes

Measurement is a key part of the optimization process. Use the
simplest algorithms and data structures you can, and measure
your code's performance to see whether you need to make
any changes. Use timing commands or profilers to locate any
bottlenecks. Optimize these hot spots one at a time, and measure
any improvement. You can use the date object to time
individual snippets:

This is useful when comparing one technique
to another. But for larger projects, only a profiler will
do. Mozilla.org includes the Venkman profiler in the Mozilla
browser distribution to help optimize your JavaScript.

The Venkman JavaScript Profiler

For more information on the Venkman profiler, see the following
web sites:

Economist Vilfredo Pareto found in 1897 that about 80 percent
of Italy's wealth was owned by about 20 percent of the
population.8 This has become the
80/20 rule or the Pareto principle, which is often applied to
a variety of disciplines. Although some say it should be adjusted
to a 90/10 rule, this rule of thumb applies to everything from
employee productivity and quality control to programming.

About the Author:
Andy King is the author of "Speed Up Your Site" and founder
of WebReference.com and JavaScript.com. As Managing Editor of
these award-winning sites, Andy became the "Usability Czar"
at internet.com. A ten-year web veteran, he has written extensively
on web site optimization; "Speed Up Your Site" is the culmination
of that work. For more information on the book and Andy's consulting
services see the companion site at http://www.WebSiteOptimization.com.
You can contact him at http://www.websiteoptimization.com/contact.